How far you can travel in a day really depends on what you are doing, what you are traveling on, whether or not you know where you're going, what you are carrying, and of course, how good of shape you're in.
Traveling day after day after day? You are going to be hard pressed to cover much more than 20 miles a day if you have anything other to do besides walk in a straight line in open land while carrying a relatively light pack.
In very rough woods where you have to blaze your own trail, find food and water, and make yourself a secure campsite in a short northern day, and all the while taking readings for mapping or something, then you might be hard pressed to make 10, or even 5. (Remember, if you have to gather fire wood and such before dark, it can really cut into how long you can actually spend traveling. Add in a lot of other tasks you have to do, and finding time to walk a full 8 hours can get hard, especially when your nights are long.)
Modern ultra light gear, where you're packing all your food as freeze-dried, high energy food, on well traveled trails that are perfectly safe to keep your eyes 10 feet in front of you and boot along at a good clip over a weekend, and a modern LED head lamp? Sure, a human in good shape could cover 50 or more miles without hardship.